Pad type hydrodynamic thrust bearings are known from the present inventor's previous U.S. Pat. Nos. 4,676,668 to Ide, 5,137,373 to Ide and 5,125,754 also to Ide. These patents each disclose movable pad thrust bearings which include spaced pads supported for deflection under load. In U.S. Pat. No. 5,137,373 the bearing can be formed in a single piece. In the other two patents, individual bearing pads are supported in a separate carrier. One of the principal advantages of these bearings is that they can be manufactured at a fraction of the cost of conventional tilt pad bearings, yet perform at least as well.
Two of the most important performance characteristics are load carrying capability which should be as large as practical and power loss due to friction which should be as small as practical. The load carrying capability of a hydrodynamic bearing and the power losses attributable to friction occurring in a hydrodynamic bearing are both directly related to film thickness. Greater film thickness results in greater load carrying capability, but also results in more friction and thus greater power consumption. In general, a thick film leads to high power losses and a thin film results in low power losses.
Film thickness is determined by the angle of attack of a pad to a shaft surface to be supported, typically a shaft runner. The angle of attack is often expressed as a ratio of distance between the leading edge and the shaft surface to be supported to the distance between the trailing edge and the shaft surface to be supported. A one to one ratio yields an angle of attack of 0.degree. and no wedge, i.e., the leading edge and the trailing edge are equally spaced from the shaft surface to be supported. In such a case, with no wedge, there is no film and poor performance results. A ratio of 1.5:1 is considered to be a small angle of attack which results in a very thin film and minimum power loss, but low load capability.
The ideal angle of attack is, of course, application dependent, but for most applications an angle of attack between 2:1 to 5:1 is best.
A ratio of 2:1 results in a medium film thickness and power loss and also results in a medium load carrying capability. A ratio of 3:1, 4:1 or 5:1 results in a higher film thickness and greater power losses, but yields higher load carrying capability.
It should also be noted that load decreases film thickness. For a high load design, i.e., 3:1 angle, that operates with low load, the film will be thick and losses high. Thus, it is normally the case that a bearing designed for a high load application is not ideally suited for low load applications. In other words, bearings should be designed for a single application or mode of operation.
To a large extent, the problems associated with prior art hydrodynamic bearings have been solved by the bearing construction described in U.S. Pat. No. 4,676,668 to Ide, the present inventor. This bearing construction includes a plurality of discrete bearing pads press fit into a support portion. The bearing pads may be spaced from the support member by at least one leg which provides flexibility in three directions. To provide flexibility in the plane of motion, the legs are angled inward to form a conical shape with the apex of the cone or point of intersection in front of the pad surface. Each leg has a section modulus that is relatively small in the direction of desired motion to permit compensation for misalignments. These teachings are applicable to both journal and thrust bearings.
While the construction described in the present inventor's previous patent represents a significant advance in the art, commercial production has shown that improvements are possible. For instance, the shape of the bearing pads is relatively complex; and consequently somewhat difficult to mass produce, use in radial or journal bearings, and dampen.
Additionally, since the bearing pads are unitary, the entire bearing pad must sometimes be constructed out of the most expensive material necessary in any part of the bearing. The unitary construction also makes it difficult to change the performance characteristics of any particular bearing pad. This necessitates a different bearing pad for each application thus limiting the ability to standardize bearing components (i.e., use standard components in different configurations for each application) and achieve the cost and other commercial advantages associated with standardization.
The press fitting of the pads into the carrier also complicates assembly of bearings. Moreover, by virtue of this press fit, the bearing pads cannot be easily removed from the carrier. This complicates reuse of the carrier (the most substantial portion of the bearing) in the event of a failure.
Also, the bearing performs optimally in only one mode of operation and its deflection characteristics are not actively controllable.
The bearings described in applicant's previous patents can be designed based upon finite element analysis to achieve virtually any desired performance under load. However, once the bearing is designed and constructed, its performance characteristics are fixed unless some provision is made to allow adjustment of the performance characteristics. For example, in U.S. Pat. No. 5,125,754, the possibility of using piezoelectric elements to vary deflection characteristics is disclosed.
Absent some structure for varying the performance of the bearings, separate bearings must be designed for each specific application or type of application. This naturally limits the ability to construct bearings of standardized parts and consequently, increases the overall cost of manufacturing bearings and lessens, to some extent, the cost advantage achieved by the bearings. For this reason, the present inventor has found that, where possible, the bearings of the present invention should be constructed using standardized parts to take advantage of economies of scale. Accordingly, there remains a need for a bearing assembly which can be constructed from simple standardized parts and which can be used in widely varying applications.